International Journal on Engineering Performance-Based Fire Codes, Volume 4, Number 4, p.106-109, 2002
CONE CALORIMETER STUDIES: EFFECT OF RADIATIVE HEAT FLUX
AND SAMPLE THICKNESS ON BURNING OF PLYWOOD SAMPLES
H.W. Au Yeung and W.K. Chow
Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China
(Received 19 April 2002; Accepted 6 May 2002)
ABSTRACT
It is necessary to study the fire behaviour of plywood as the material is widely used for partitioning places into
rooms. For example, most of the karaoke boxes built six years ago are made of plywood boards. In this paper,
studies on burning behaviour of plywood samples by a cone calorimeter are reported. From the results, ignition
time and time to first peak for different incident heat fluxes and samples thickness are discussed.
It is observed from this study that shorter ignition time was found under higher heat flux, even for thicker
plywood samples. For example, by increasing the heat flux from 20 kWm-2 to 50 kWm-2 on plywood of 6 mm
thick under ambient condition, the ignition time reduced from 191 s to 13 s. The ignition time decreased from
191 s to 152 s when the thickness of plywood sample was increased from 6 mm to 18 mm.
1.
INTRODUCTION
As reviewed before [e.g. 1], timber products
including plywood, are commonly used as partition
materials.
Many places, including karaoke
establishments opened six years ago, might have
boxes made of plywood boards. By that time,
using gypsum as plaster board with adequate fire
protection were not yet popular.
Therefore,
studying the burning behaviour of plywood is
important for safety assessment.
Cone calorimeter is now an “essential tool” in
studying transient heat release rate for materials of
products and assemblies [2,3] using the oxygen
consumption method. In addition, optical smoke
density, mass loss rate, concentrations of carbon
monoxide and carbon dioxide, and amount of soot
produced can also be measured with optional
equipment provisions. This has led to a number of
studies aimed at identifying the factors responsible
for the heat release rate of different materials and
assessing their suitability for use in the
constructions, textiles and furniture industries [2-5].
There are other bench-scale testing methods to give
heat release rate which are suitable for studying
wood-plastic composites [e.g. 6]. An example is
the Ohio State University (OSU) test apparatus [7].
The objective of this paper is to study the fire
behaviour of plywood with a cone calorimeter.
Correlations between ignition time, time to first
peak, and peak value of HRR with the incident heat
flux and sample thickness are derived.
Experiments isolating the effects due to individual
factors on the heat release rate curves of plywood
product were performed. Incident heat flux is an
obvious influence factor. In addition, the effect of
the sample thickness cannot be ignored. Measured
results would provide information for better
understanding on the fire behaviour of materials.
Factors which are important in the development of
fire codes can then be identified.
A series of experimental measurements were
conducted with the cone calorimeter.
Key
operational parameters such as input heat flux and
thickness of specimen or curve were varied
independently. Unpainted plywood samples were
cut into squares of 10 cm by 10 cm and tested
under incident heat flux Rf from 20 kWm-2 to 50
kWm-2. The sample thickness of specimen DS
varied from 6 mm to 18 mm.
2.
IGNITION TIME
How Rf in the cone calorimeter can be adjusted was
described clearly in the literature [2,8]. Rf acting
on the sample depends on:
y
y
y
Temperature of the cone heater
Distance between the cone and the sample
Humidity and temperature of the environment
The distance between the cone and the sample was
fixed in the tests. Temperature of the electric
conical heater was determined by the temperature
controller. Incident heat flux was calibrated by the
cone temperature. The apparatus and samples were
kept under ambient environment of similar
humidity and room temperature. Therefore, the
above three points can be kept to give consistent
value of Rf.
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International Journal on Engineering Performance-Based Fire Codes
Effect of flux Rf on the ignition time tig is
illustrated in Fig. 1. Shorter ignition time was
found for higher incident heat flux. With specimen
of 6 mm thick, the ignition time reduced from 191 s
to 13 s when the input heat flux was increased from
20 kWm-2 to 50 kWm-2. Similar results were
observed for samples of thickness 12 mm and 18
mm, although shorter tig were recorded.
observed for thicker plywood. For Rf at 20 kWm-2
and sample thickness increased from 6 mm to 18
mm, the required ignition time decreased from 191
s to 152 s. Similar sets of results were obtained for
Rg of 30, 40 and 50 kWm-2.
Therefore, ignition time for combustible materials
would be decreased if its thickness is increased. A
thicker sample would have a longer ignition time.
The effect of sample thickness DS on tig at different
Rf is shown in Fig. 2. Shorter ignition times were
6 mm
12 mm
Ignition time tig / s
18 mm
Incident heat flux Rf / kWm-2
Fig. 1: Effect of incident heat flux on ignition time
Ignition time tig / s
20 kWm-2
30 kWm-2
40 kWm-2
50 kWm-2
Thickness DS / mm
Fig. 2: Effect of sample thickness on ignition time
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International Journal on Engineering Performance-Based Fire Codes
3.
TIME TO FIRST PEAK OF HEAT
RELEASE RATE
Similar patterns were observed for sample
thickness from 12 mm to 18 mm. However, a
longer tp was obtained with thicker samples as
shown in Fig. 4.
Time to first peak of heat release rate tfp / s
Measured times tfp required to reach the first peak
of heat release rate are plotted against Rf in Fig. 3.
It is observed that values of tfp were fairly constant
over a range of Rf. For the 6 mm thick plywood
sample, tfp for Rf of 20 kWm-2 and 50 kWm-2 were
29 s and 18 s respectively.
It is reported [5,8,9] that tfp is related to the rate of
fire growth. Results indicated that the transition of
a typical room fire resulted by burning this
plywood sample to flashover would not depend on
Rf.
18 mm
12 mm
6 mm
Input heat flux Rf / kWm-2
Time to first peak of heat release rate tfp / s
Fig. 3: Effect of incident heat flux on time to first peak of heat release rate
30 kWm-2
40 kWm-2
20 kWm-2
50 kWm-2
Thickness DS / mm
Fig. 4: Effect of sample thickness on time to first peak of heat release rate
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International Journal on Engineering Performance-Based Fire Codes
4.
CONCLUSIONS
Within the range of this study, the experimental
measurements on the plywood sample illustrated
that:
y
Higher the incident heat flux, shorter the
ignition time.
y
Thicker the sample, shorter the ignition time.
y
The time to first peak of heat release rate does
not depend on the incident heat flux.
However, a longer time is required to reach
the first peak of heat release rate with thicker
plywood samples.
ACKNOWLEDGEMENTS
The project is supported by the Advanced
Buildings Technology in a Dense Urban
Environment, Area of Strategic Development,
Faculty of Construction & Land Use, The Hong
Kong Polytechnic University.
NOMENCLATURE
Rf
DS
tig
tfp
Incident radiative heat flux, kWm-2
Sample thickness, mm
Ignition time, s
Time to first peak value, s
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